A selective catalytic reduction NOx catalyst (hereinafter, also simply referred to as a “NOx catalyst”) is known which uses ammonia as a reducing agent to purify NOx (nitrogen oxides) contained in exhaust gas from an internal combustion engine. A supply apparatus for supplying ammonia or an ammonia precursor such as urea (hereinafter, also collectively referred to as a “reducing agent”) into exhaust gas that flows into the NOx catalyst may be installed on an upstream side of the NOx catalyst in an exhaust passage. In order to cause NOx to be purified at a desired purification rate at the NOx catalyst, a supply amount of the reducing agent supplied from the supply apparatus must be controlled. A NOx sensor which is provided on a downstream side of the NOx catalyst and which is capable of detecting a concentration of NOx in exhaust gas is sometimes used for this control. Regarding the control of a supply apparatus, it is known that, when an output value from a NOx sensor follows a downward trend regardless of an increase in an amount of NOx flowing into a NOx catalyst, a determination is made that ammonia is flowing out from the NOx catalyst (an ammonia slip has occurred) due to an excessive supply amount of a reducing agent and, accordingly, an additive amount of urea water is reduced (for example, refer to PTL 1).
Accurate control of an amount of a reducing agent supplied from a supply apparatus requires the supply apparatus to be operating properly. Since a NOx sensor is capable of detecting ammonia in addition to NOx, an output value from a NOx sensor is known to be used to diagnose whether or not a supply apparatus is operating properly (in other words, whether the supply apparatus is in a normal state or an abnormal state) (for example, refer to PTL 2 through PTL 4). According to the techniques disclosed in these documents, during a diagnosis of a supply apparatus, a supply amount of a reducing agent from the supply apparatus is increased to a level where an output value of the NOx sensor is expected to exceed a prescribed threshold due to ammonia flowing out from the NOx catalyst. In addition, when an actual output value does not exceed the prescribed threshold, it is determined that the supply apparatus is not operating according to control and is in an abnormal state.